How feasible would a nuclear thermal plasma jet rocket be?

[Spacescifi]

So I have heard of scram and ram jets. Ideally you would have an SSTO rocket capable of doing both, due to changes in airflow effecting thrust.

Would it be better to have both scram and ram jet ability and a nuclear thermal reactor for plasma jets?

Or would it be possible to get by using a limited fuel scram/ram jet combo? The only advantage would be less weight

 Yet not being able to use air itself as your only propellant is an advantage only the nuclear reactor has, even though it makes it heavier. Also it can switch to combustion rockets when the aor becomes too thin.

 

What do you think?

Edited June 5, 2019 by Spacescifi

[ARS]

For nuclear thermal plasma jet rocket concept, first, we need to evaluate the feasibility of nuclear engine itself. Nuclear propulsion for aircraft has already tested during cold war by US and USSR using their nuclear aircraft program, with the aim to create a strategic bomber using the very air itself as propellant for unmatched operational range. Both program didn't have a promising result, with different reason from each side.

For US, the main problem is the crew shielding, since a nuclear reactor and associated equipment for cooling and radiation shielding is very heavy, greatly limiting the bomb load capacity, and also reducing it's maximum speed. Another is the maintenance problem since the maintenance of nuclear engines are very demanding compared to normal aircraft engine. USSR managed to get away with the issue of radiation shielding by shielding the crew instead of the reactor, but this makes the aircraft much more radioactive than US, and thus requires regular decontamination operation to keep it usable. I've written an article about this maintenance difficulties in this forum, please take a look (it's a very interesting read): 

 

Ultimately, the programs did not produce a promising result as the shielding and maintenance problems plagued the prototypes, and once the practical ICBM came into service, the operational range of nuclear aircraft becomes a moot point since it does not justify the environmental disaster the nuclear aircraft caused if it's shot down as well as placing the crew at risk during the mission just to send a couple of nuclear bombs compared to ICBM method. With the reasons above, both projects are scrapped. As such, the full potential of nuclear aircraft itself is still not fully understood yet

[Spacescifi]

16 minutes ago, ARS said:

For nuclear thermal plasma jet rocket concept, first, we need to evaluate the feasibility of nuclear engine itself. Nuclear propulsion for aircraft has already tested during cold war by US and USSR using their nuclear aircraft program, with the aim to create a strategic bomber using the very air itself as propellant for unmatched operational range. Both program didn't have a promising result, with different reason from each side.

For US, the main problem is the crew shielding, since a nuclear reactor and associated equipment for cooling and radiation shielding is very heavy, greatly limiting the bomb load capacity, and also reducing it's maximum speed. Another is the maintenance problem since the maintenance of nuclear engines are very demanding compared to normal aircraft engine. USSR managed to get away with the issue of radiation shielding by shielding the crew instead of the reactor, but this makes the aircraft much more radioactive than US, and thus requires regular decontamination operation to keep it usable. I've written an article about this maintenance difficulties in this forum, please take a look (it's a very interesting read): 

 

Ultimately, the programs did not produce a promising result as the shielding and maintenance problems plagued the prototypes, and once the practical ICBM came into service, the operational range of nuclear aircraft becomes a moot point since it does not justify the environmental disaster the nuclear aircraft caused if it's shot down as well as placing the crew at risk during the mission just to send a couple of nuclear bombs compared to ICBM method. With the reasons above, both projects are scrapped. As such, the full potential of nuclear aircraft itself is still not fully understood yet

Thanks.

Assuming mankind went full Kerbal on this (commit to building it anyway), and nations allowed it for once, I think it could work.

 

BTW.... I had no intention of it shipping crew. I intended for it to be a cargo shipping rocket to the ISS or whatever orbiting spacecraft we have up there.

Basically send a rocket up with ONLY crew, then supply it with cargo from the SSTO cargo rocket which follows it up to orbit.

[satnet]

The Achilles heel of most of these air breathing engines (ram/scram jets) has been that their operating range it too narrow to go from 0 to orbital or even a significant fraction (by everyday flight standards they cover a huge range, but space flight requires 0 to 7.7 km/s which is a great deal larger). Within their operating range they have a higher overall propulsive efficiency, but to cover the whole range you need multiple engine types which means more mass for engines and less for payload often to the point of being sub-orbital. There are proposed dual-mode scram jets that can work at lower speeds, though I don't think they've been designed or demonstrated and I think they just get into the ram jet territory not down to subsonic ranges where turbofans dominate.

Nuclear rockets tend to have a low thrust to weight ratio which usually means they are limited in the design to space or late stage sub-orbital where you need delta-V more than thrust. Of course this is also a problem for scram jets so you now have to low thrust to weight ratio engines which is a real problem. You might be able to use the nuclear reactor as the heat source for a dual-mode scram jet, which would at least help keep the weight down assuming the heat transfer mechanism isn't as heavy as the fuel injection system (and it could very well be as heavy or heavier negating the benefit). Of course as long as we're talking fission and not fusion, as @ARS pointed out you have the environmental and shielding issues to deal with.

Isaac Arthur just did an episode on space planes and talks about ram, scram jets and a few other things. He has other episodes on nuclear rockets, though it has been long enough I don't recall if any would be relevant.

 

[RocketSquid]

There are two vehicles I would like to call your attention to:

1. The SLAM/Flying Crowbar: https://en.wikipedia.org/wiki/Supersonic_Low_Altitude_Missile

This little terror would fly around at treetop heights using a nuclear ramjet, meaning the sonic boom it created would be actively dangerous. The engine was entirely unshielded, but chances are that wouldn't have mattered. If it crashed, or rather when it crashed, seeing as it couldn't land or be refueled, it would be a dirty bomb, spreading nuclear waste over a wide area as it collided into the ground at mach 4. The US abandoned the project, fortunately, but not before testing the nuclear ramjet using compressed air. So the idea of a nuclear ramjet is certainly not implausible.

2. The Ayaks Hypersonic Waverider: https://en.wikipedia.org/wiki/Ayaks

The Ayaks is a unique hypersonic craft that, rather than using ramjets or scramjets, can use modified turbojets even at high speeds, and can operate at altitudes beyond even that of scramjets. It does this by electromagnetically compressing the intake air beforehand, then electromagnetically accelerating the plasma exhaust afterwards. If you want maximum efficiency, you can use turbojets for most of the flight envelope and rockets at the very end, but for greater simplicity and lower mass you can use rockets for takeoff and getting up to sufficient speed for the ramjet to take over. 

In theory, you could combine these two ideas. However, I don't think the use of a nuclear engine in such a vehicle is practical. Other than the risk of crashing, there's the fact that most if not all hypersonic vehicles need active cooling, so that will always limit your flight time and eat up your mass budget.

[KerikBalm]

5 hours ago, RocketSquid said:

The US abandoned the project, fortunately, but not before testing the nuclear ramjet using compressed air. So the idea of a nuclear ramjet is certainly not implausible.

Well, I was going to mention project pluto, but that0s basically the same thing (Pluto was to lead into SLAM)

https://en.wikipedia.org/wiki/Project_Pluto

Now, I don't know how easy it would be to design the reactor for two differnet working fluids, with different neutron moderating properties and mass flow rates... but one could conceptually imagine taking pluto, closing the intake, and pumping LH2 through it. Of course, pluto needed to be accelerated to work (like any ramjet)...

Now if we further imagine a NTR as developed into a LANTR http://www.projectrho.com/public_html/rocket/enginelist2.php#lantr

Then we can get a high TWR and pretty good ISP that maybe could boost it fast enough to pump air around the reactor as a ramjet, or at least an air augmented rocket, and SSTO to orbit (shutting off LOX thrust augmentation once enough propellent is expended, and the thrust from hydrogen alone is sufficient)

 

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2. The Ayaks Hypersonic Waverider: https://en.wikipedia.org/wiki/Ayaks

I am very skeptical of that

[RocketSquid]

11 hours ago, KerikBalm said:

I am very skeptical of that

There have been quite a few studies into the feasibility of the Ayaks concept, and the general conclusion is that it would work, though probably not as well as the russians say it would.

[DDE]

18 hours ago, Spacescifi said:

BTW.... I had no intention of it shipping crew. I intended for it to be a cargo shipping rocket to the ISS or whatever orbiting spacecraft we have up there.

That only slightly diminishes the shielding requirements. Electronics don’t like knocked-out bits, mechanical payloads don’t like neutron embrittlement, and food doesn’t taste well with free radicals.

12 hours ago, KerikBalm said:

I am very skeptical of that

Any “super-secret Soviet tech” surfacing in the 1990s is suspect.

21 hours ago, Spacescifi said:

and a nuclear thermal reactor for plasma jets

Please define. It sounds suspiciously like a nuclear-electric powerplant.

[KerikBalm]

10 hours ago, RocketSquid said:

There have been quite a few studies into the feasibility of the Ayaks concept, and the general conclusion is that it would work, though probably not as well as the russians say it would.

It kind of reminds me of this:

https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive

but for the air instead of water. Technically possible, but inefficient, and impractical, inferior to other alternatives.

Its also vaguely reminiscent of this:

https://en.wikipedia.org/wiki/Atmosphere-breathing_electric_propulsion

which is more practical.

The idea that you'll use a magnetic air intake with ionized air, instead of a physical scoop, seems... like its going to use a lot of energy and be inefficient. The using EM forces to accelerate the exhaust... where is all this energy coming from? where is the generator, how much does it weight, what is the TWR?

To quote one of the .pdfs the wikipedia article cites:

Quote

The weak, non-equilibrium ionization is provided by an external electrical means such as an electron beam.

...

The specific MHD inlet design under study is an annular generator/accelerator concept (patent pending). Its embodiment has a superconducting magnet in a configuration similar to that of a Hall Thruster for spaceapplications.

 

I doubt this will be practically competitive against a scramjet/sabre rocket, etc.

 

Now, we do have working air breathing ion engines, but they are for very high orbit, where thrust, drag, and power requirements are very low. Its also to maintain speed7counteract drag, not provide meaningful acceleration over the short term.

[RocketSquid]

1 hour ago, KerikBalm said:

It kind of reminds me of this:

https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive

but for the air instead of water. Technically possible, but inefficient, and impractical, inferior to other alternatives.

Its also vaguely reminiscent of this:

https://en.wikipedia.org/wiki/Atmosphere-breathing_electric_propulsion

which is more practical.

The idea that you'll use a magnetic air intake with ionized air, instead of a physical scoop, seems... like its going to use a lot of energy and be inefficient. The using EM forces to accelerate the exhaust... where is all this energy coming from? where is the generator, how much does it weight, what is the TWR?

To quote one of the .pdfs the wikipedia article cites:

I doubt this will be practically competitive against a scramjet/sabre rocket, etc.

 

Now, we do have working air breathing ion engines, but they are for very high orbit, where thrust, drag, and power requirements are very low. Its also to maintain speed7counteract drag, not provide meaningful acceleration over the short term.

The energy comes from the air, and ultimately from the engine. Since the intake scoop is decelerating the air, it generates large amounts of electricity, which is used to ionize the air and to re-accelerate it. The energy used to ionize the air is partially reused in that the ionized air is more chemically reactive than non-ionized air. The main advantage of this system over scramjets is that the use of a magnetic scoop extends the altitude ceiling of the air-breathing engines, and the use of turbojets means the air-breathing portion is even more efficient. As for scramjets and SABRE, I frankly don’t think either will be practical without either hydrocarbon fuel (for the scramjet) or slush hydrogen.

Edited June 6, 2019 by RocketSquid

[KerikBalm]

5 minutes ago, RocketSquid said:

Since the intake scoop is decelerating the air, it generates large amounts of electricity

Decelerating Ions will generate electricity, but it also creates large amounts of drag

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, which is used to ionize the air and to re-accelerate it.

At far less than 100% efficiency

Quote

The energy used to ionize the air is partially reused in that the ionized air is more chemically reactive than non-ionized air.

Reactivity and chemical energy aren't the same thing. When you ionize the air, the ions are positively charged, and the electrons are negatively charged. You're going to be losing a lot of energy in the form of those free electrons which will be directed away from the ions by your magnetic fields.

Quote

The main advantage of this system over scramjets is that the use of a magnetic scoop extends the altitude ceiling of the air-breathing engines,

Why, cause you use a magnetic scoop? why not just a bigger physical scoop?

Quote

and the use of turbojets means the air-breathing portion is even more efficient.

Ummm, and why is that? the advantage of a jet, turbo or otherwise, is in pushing more mass at lower velocity (for the same energy it takes to accelerate 1 kg of reaction mass to 10,000 m/s, you can accelerate 100 kg of reaction mass to 1,000 m/s for 10x the thrust). Turbo, Ram, Scram, what matters is essentially the ratio of the mass of accelerated air to the mass of fuel burned... similar to bypass ratio.

Quote

As for scramjets and SABRE, I frankly don’t think either will be practical without either hydrocarbon fuel (for the scramjet) or slush hydrogen.

They seem pretty practical to me. They have working flying examples, unlike this drive.

back to the wikipedia article, with emphasis;

Quote

Few large-scale working prototypes have been built, as marine MHD propulsion remains impractical due to its low efficiency, limited by the low electrical conductivity of seawater. Increasing current density is limited by Joule heating and water electrolysis in the vicinity of electrodes, and increasing the magnetic field strength is limited by the cost, size and weight (as well as technological limitations) of electromagnets and the power available to feed them.^[14][15]

Stronger technical limitations apply to air-breathing MHD propulsion (where ambient air is ionized) that is still limited to theoretical concepts and early experiments.

 

[RocketSquid]

Just now, KerikBalm said:

Decelerating Ions will generate electricity, but it also creates large amounts of drag

At far less than 100% efficiency

Reactivity and chemical energy aren't the same thing. When you ionize the air, the ions are positively charged, and the electrons are negatively charged. You're going to be losing a lot of energy in the form of those free electrons which will be directed away from the ions by your magnetic fields.

Why, cause you use a magnetic scoop? why not just a bigger physical scoop?

Ummm, and why is that? the advantage of a jet, turbo or otherwise, is in pushing more mass at lower velocity (for the same energy it takes to accelerate 1 kg of reaction mass to 10,000 m/s, you can accelerate 100 kg of reaction mass to 1,000 m/s for 10x the thrust). Turbo, Ram, Scram, what matters is essentially the ratio of the mass of accelerated air to the mass of fuel burned... similar to bypass ratio.

They seem pretty practical to me. They have working flying examples, unlike this drive.

back to the wikipedia article, with emphasis;

 

The magnetic scoop is largely only activated at very high altitudes, where drag is less significant, the magnetic scoop is needed because the weight of a physical scoop would be immense and it would have no efficient way of decelerating the intake air without also generating immense amounts of heat. No scramjet aircraft has ever flown for more than three minutes under power.

[KerikBalm]

I don't see how a magnetic scoop wouldn't also generate immense amounts of heat... if its actually scooping up significant amounts of air slowing it down to subsonic speeds (relative to the aircraft's engine), and compressing it by a whole lot.

[RocketSquid]

31 minutes ago, KerikBalm said:

I don't see how a magnetic scoop wouldn't also generate immense amounts of heat... if its actually scooping up significant amounts of air slowing it down to subsonic speeds (relative to the aircraft's engine), and compressing it by a whole lot.

The heat would still be generated but less of it would be conducted into the body of the craft